This invention relates to the field of power controls for AC induction motors. More particularly, it relates to the field of power factor control circuits for such motors, the intent of which is to realize energy savings by optimizing the power factor of the motor during normal operation. As is well known, an AC induction motor has its optimum power factor under full load, constant voltage conditions. Many times, however, a motor is operating at other than full load, constant voltage conditions and, accordingly, the power factor is reduced resulting in increased operating cost. In part this is because the motor draws essentially the same current whether fully loaded, partially loaded or unloaded. Therefore, the motor efficiency is less when the rated load is not present.
Because of this situation it is desirable to provide a power factor control circuit for use in conjunction with motors which are frequently operated at other than their rated load. Thus, for example, in the case of a machine tool, such as a drill press, the motor which operates the drill must be large enough to produce the torque imposed during operation of the drill. This full load condition may last for several seconds after which the motor will run in an unloaded condition for significant periods of time before the next drill operation. In such situations, as previously indicated, power factor control can provide significant cost savings and improved efficiency.
Power factor control for AC induction motors is known and, indeed, many circuits have been developed for that purpose. As indicated in the prior art statement, one such prior art control system is disclosed and claimed in U.S. Pat. No. 4,052,648. In that system power factor control is achieved by sampling the line voltage and the current of the induction motor. The samples are compared and a control or error signal representative of their phase difference is obtained. This error signal is employed to vary the duty cycle (on time portion of each AC cycle) for the motor thereby to regulate the phase difference and improve the power factor. In that system, however, control of the duty cycle is a function of the line voltage and hereinafter will be referred to as a voltage delay system.
In theory the prior art power factor control systems of the type disclosed in U.S. Pat. No. 4,052,648, are capable of achieving meaningful improvements in power factor. In practice, however, these systems have not been entirely satisfactory. They fail to account for inherent problem present in AC induction motors, namely, the current through the motor varies even when the line voltage remains constant.
This is due to the impedance characteristics of the motor which is a time variable inductor. Due to its rotation, the motor is inherently variable in its characteristics. Thus, the current through the motor oscillates and causes small speed changes and a corresponding loss of energy savings if not properly controlled. This characteristic, in large part, defeats the energy savings obtained by the prior art power factor control circuits. The losses take the form of motor vibration, excessive operating temperatures, noise, and a possible increase in maintenance requirements.
In the prior art the only way to try and stabilize the power factor circuit against such motor current instability was to utilize large capacitors in the low pass filter section of the circuit to make system response slow. This approach is somewhat disadvantageous for many industrial applications because of the slow response time obtained. Often a quick response system is necessary as, for example, in industrial sewing machines or other machines tools.
Accordingly, it is a principal object of the invention to provide a power factor control circuit which can account for these vagaries in motor current while still obtaining the benefits of power factor control.
It is another object of the invention to provide a power factor control circuit which is relatively insensitive or immune to current variation through the motor while still obtaining all the benefits of power factor control.
A further object of the invention is to provide a power factor control circuit which may be characterized as a current delay system in which the duty cycle employed for power factor control is derived from the motor current.
Another object of the invention is to provide a power factor control circuit with improved energy savings capability by use of a logarithmic feedback circuit.
These and other objects of the invention will become apparent from the detailed description which follows taken in conjunction with the drawings.